The present invention relates to a process for monitoring the cure of a binder, particularly an isocyanate-terminated prepolymer binder, during the production of engineered or composite lignocellulosic (e.g., wood) products and to an apparatus useful therefor.
Composite wood products such as oriented strand board, particle board and flake board are generally produced by blending a binder composition with wood flakes, wood strips or strands, pieces of wood or other comminuted lignocellulosic materials. This blended composition is then typically formed into a mat which is compressed between heated platens or plates to set the binder and bond the flakes, strands, strips, pieces, etc. together in densified form. Conventional processes are generally carried out at temperatures of from about 150 to about 205.degree. C. Steam is used when the board thickness prohibits the transfer of heat from the press platens to the center of the board. Steam injection during the press closing cycle enables the center of the board to be preheated before the press is closed and ensures a complete cure throughout the thickness of the board. The conventional processes also generally require that the moisture content of the lignocellulosic material be between 2 and 8% before it is blended with the binder.
Binder compositions which have been used in making such composite wood products include phenol formaldehyde resins, urea formaldehyde resins and isocyanates. See, for example, James B. Wilson's paper entitled, "Isocyanate Adhesives as Binders for Composition Board" which was presented at the symposium "Wood Adhesives-Research, Applications and Needs" held in Madison, Wis. on Sep. 23-25, 1980, in which the advantages and disadvantages of each of these different types of binders are discussed.
Isocyanate binders are commercially desirable because they have high adhesive and cohesive strength, flexibility in formulation, versatility with respect to cure temperature and rate, excellent structural properties, the ability to bond with lignocellulosic materials having high water contents, and no formaldehyde emissions. The disadvantages of isocyanates are difficulty in processing due to their high reactivity, adhesion to platens, lack of cold tack, high cost and the need for special storage.
Isocyanate prepolymers are among the preferred isocyanate materials which have been used in binder compositions to solve various processing problems, particularly adhesion to press platens and high reactivity. U.S. Pat. No. 4,100,328, for example, discloses isocyanate-terminated prepolymers which improve product release from a mold. U.S. Pat. No. 4,609,513 also discloses a process in which an isocyanate-terminated prepolymer binder is used to improve product release. A binder composition in which a particular type of isocyanate prepolymer is used to improve adhesiveness at room temperature is disclosed in U.S. Pat. No. 5,179,143.
A major processing difficulty encountered with isocyanate binders is the rapid reaction of the isocyanate with water present in the lignocellulosic material and any water present in the binder composition itself. One method for minimizing this difficulty is to use only lignocellulosic materials having a low moisture content (i.e., a moisture content of from about 3 to about 8%). This low moisture content is generally achieved by drying the cellulosic raw material to reduce the moisture content. Such drying is, however, expensive and has a significant effect upon the economics of the process. Use of materials having low moisture contents is also disadvantageous because panels made from the dried composite material tend to absorb moisture and swell when used in humid environments.
Another approach to resolving the moisture and isocyanate reactivity problem is disclosed in U.S. Pat. No. 4,546,039. In this disclosed process, lignocellulose-containing raw materials having a moisture content of up to 20% are coated with a prepolymer based on a diphenylmethane diisocyanate mixture. This prepolymer has a free isocyanate group content of about 15 to about 33.6% by weight and a viscosity of from 120 to 1000 mPa.multidot.s at 25.degree. C. This prepolymer is prepared by reacting (1) about 0.05 to about 0.5 hydroxyl equivalents of a polyol having a functionality of from 2 to 8 and a molecular weight of from about 62 to about 2000 with (2) one equivalent of a polyisocyanate mixture containing (a) from 0 to about 50% by weight of polyphenyl polymethylene polyisocyanate and (b) about 50 to about 100% by weight isomer mixture of diphenylmethane diisocyanate containing 10-75% by weight of 2,4'-isomer and 25-90% by weight of 4,4'-isomer.
U.S. Pat. No. 5,002,713 discloses a method for compression molding articles from lignocellulosic materials having moisture contents of at least 15%, generally from 15 to 40%. In this disclosed method, a catalyst is applied to the lignocellulosic material. A water-resistant binder is then applied to the lignocellulose with catalyst and the coated materials are then compression shaped at a temperature of less than 400.degree. F. to form the desired composite article. The catalyst is a tertiary amine, an organometallic catalyst or a mixture thereof. The binder may be a hydrophobic isocyanate such as any of the polymeric diphenylmethane diisocyanates, m- and p-phenylene diisocyanates, chlorophenylene diisocyanates, toluene diisocyanates, toluene triisocyanates, triphenylmethane triisocyanates, diphenylether-2,4,4'-triisocyanate and polyphenol polyisocyanates. The catalyst is included to ensure that the isocyanate/water reaction is not slowed to such an extent that the pressing time necessary to produce the molded product is significantly increased.
Pressing of wafer board, oriented strand board, and parallel strand lumber using steam injection and a conventional binder such as a urea-formaldehyde resin or a polymeric diphenylmethane diisocyanate (MDI) is known. Examples of such known pressing processes are disclosed in U.S. Pat. Nos. 4,684,489; 4,393,019; 4,850,849; and 4,517,147.
The processes disclosed in U.S. Pat. Nos. 4,517,147 and 4,684,489 require pressing and steaming of wood mat in several stages. First, the air present in the mat is driven out. The wood strands are then softened to allow compression to a higher density and the center of the mat is preheated to a uniform temperature. It is apparent from these disclosures that resin cure must be monitored and the press closing and steaming cycles must be controlled to minimize the amount of resin prematurely cured before the press is fully closed and to optimize board properties.
The completeness of binder cure may, of course, be determined by destructive testing of samples which have been permitted to cure for varying amounts of time under the process conditions. The cure time to be used during the production process is determined on the basis of the sample which had completely cured in the least amount of time. The disadvantages of this method are readily apparent. Valuable product is destroyed in the testing. Further, any variation in wood composition, extent of binder dispersion on the wood particles, etc. or processing conditions which would affect the rate of binder cure are not taken into consideration in the above-described method.
Those in the art have therefore continued to seek a reliable, non-destructive method for monitoring the cure of the binder in composite materials during the production process.
One approach which is considered to be promising is the use of ultrasonic waves to determine the viscosity or some other dynamic property of the binder. Johnson et al discuss the theory behind use of ultrasonic waves in such applications in their proposal entitled "Acoustic Measurement of the Mechanical Properties of Thin Material Specimens" in Aerospace Report # TR-0091(6935-08)-1.
In U.S. Pat. No. 5,009,104, the viscosity of a composite part as it is cured is measured. In this disclosed method, the composite material is secured in a vacuum bag and carried by a tool having an aperture which accommodates an acoustic wave guide coupled directly to the part. Pulses of ultrasound energy are directed through the wave-guide and the amplitude of the reflected pulses is monitored. It is not possible by this method to determine viscosity change at any point of the composite article which is not within the aperture of the tool. This method is effective when a homogeneous, semi-liquid resin that will conform to the shape of the tool and allow efficient coupling of the acoustic wave between the tool and composite resin is used. However, in the production of engineered lumber, particularly oriented strand board, the wood mat is not homogeneous. This lack of homogeneity causes a multitude of scattered acoustic waves that when recombined at the receiving transducer generate a complex signal that changes shape and amplitude. Scattered waves should be eliminated because they interfere with the measurement of reflected pulse amplitude. The method disclosed in U.S. Pat. No. 5,009,104 will not therefore be effective for monitoring non-homogeneous media.
In their article entitled "Acoustic Monitoring of Cold-Setting Adhesive Curing in Wood Laminates: Effect on Clamping Pressure and Detection of Defective Bonds" which appeared in Wood and Fiber Science, 28(1), 1996, pp. 7-14, Biernacki et al report the results of their study of an ultrasonic method for monitoring the bonding process and assessing the quality of cured bonds in wood laminates. Monitoring was conducted at normal and at angular (5.degree.) incidence to the bond plane. It was found that defective bonds could be detected using patterns of relative attenuation changes during curing. Selection of optimum clamping pressure could be made based upon the observation that transmission of the ultrasonic waves through uncured bond lines was strongly affected by pressure.
In their article entitled "Acoustic Monitoring of Cold-Setting Adhesive Curing in Wood Laminates" which appeared in Int. J. Adhesion and Adhesives, Vol.16, No.3 (1996), pp.165-172, Biernacki et al report that acoustic transmission is sensitive to different bond types and curing phases. It is also reported that a reasonable correlation between the relative coefficient of transmission and the development of strength over time was found. The conclusions stated in this article are, however, based upon the results of tests conducted under very strictly controlled conditions.
The tightly controlled conditions described in the Biernacki et al articles could not, however, be maintained during commercial production of composite wood materials. The Biernacki et al methods are based on a single bond interface. In commercial composite wood production processes, there are a large number of bond interfaces. The scattering of acoustic waves in a non-homogeneous medium such as wood mats would render the Biernacki et al methods ineffective.
In "Ultrasonic Cure Monitoring of Advanced Composites", David D. Shepard et al, published in the proceedings of the 42nd International SAMPE Symposium, May 4-8, 1997, Shepard et al describe a method and apparatus for monitoring the cure of thermosetting resins and composites in which the speed of ultrasonic acoustic waves through the resin or composite is measured. Shepard et al reports that there is a good correlation between the ultrasonic method disclosed therein and the widely used dielectric cure monitoring method. The Shepard et al method requires ultrasonic transducers mounted in the walls of the press. This method may be impractical for existing production presses in which the press would have to be disassembled to mount the transducers. In addition, when the steam press cycle is used, the acoustic noise generated by the high-pressure steam passing through the press platens will generally override the ultrasonic signal from the transducer.